Diffusion behavior of selenite in purified bentonite

Selenium (Se) is an important element for assessing the safety of high-level waste disposal. In this study, the diffusion coefficients of selenite in purified bentonite were investigated. A Japanese purified sodium bentonite, Kunipia-F, which contains approximately 99 wt% montmorillonite, was used in this experiment. Bentonite powder was compacted into cylinders (diameter, 10 mm; height, 10 mm; dry density, 0.8–1.6 Mg m⁻³). Each compacted bentonite specimen was inserted into an acrylic resin column and saturated with 0.01 M–1.0 M of NaCl (aq.) for 30 days. One side of the bentonite was spiked with 10 μL of tracer solution containing 0.13 M Na₂SeO₃ and it was kept at 10, 25, 40, or 55 °C for 1–29 days. The apparent diffusion coefficients of selenite in compacted bentonite were 2.5 × 10⁻¹¹ to 1.9 × 10⁻¹³ m² s⁻¹ and decreased with the increase in dry density. The ionic strength showed no significant effect on the apparent diffusion coefficient of selenite for bentonite densities higher than 1.4 Mg m⁻³. However, the apparent diffusion coefficients for 1 M NaCl were several times larger than those in 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m⁻³. This increase in diffusion coefficient would be caused by a change in anion accessible porosity. The apparent diffusion coefficient would be explained by Archie's law and is proportional to exponentiation of porosity. It is desired to measure exact anion accessible porosity. The activation energy of selenite in purified bentonite was 20 ± 4 kJ mol⁻¹ and was similar to the activation energies of oxo-anions in free water. This also indicates that selenite diffuses in free pore water in the bentonite.